1. Field of the Invention
The present invention relates to an ultrasonic image processing apparatus which generates an image indicating the motion information of a subject to be examined on the basis of echo information obtained by transmitting/receiving ultrasonic waves to/from the subject, and a control program for the ultrasonic image processing apparatus.
2. Description of the Related Art
There is known an ultrasonic diagnostic apparatus which scans the interior of a subject with ultrasonic waves and visualizes the interior of the subject on the basis of reception signals generated from reflected waves from the interior of the subject. This ultrasonic diagnostic apparatus transmits ultrasonic waves from an ultrasonic probe into a subject, and receives, by the ultrasonic probe, reflected waves generated in the subject owing to acoustic impedance mismatch, thereby generating reception signals. It is known that such an ultrasonic diagnostic apparatus is also useful for cardiac diagnosis. Above all, it is very useful for diagnosis to objectively and quantitatively evaluate the function of a living tissue such as cardiac muscle.
As a therapy which has recently attracted attention, the cardiac resynchronization therapy (CRT) for a patient with severe heart failure is available. Attempts have been made to use an ultrasonic diagnostic apparatus for quantitative evaluation to determine whether this cardiac resynchronization therapy can be applied and determine the effects of the therapy.
This cardiac resynchronization therapy will be briefly described below. Many patients with severe heart failure also have systolic dyssynchrony of cardiac wall motion. The heart moves owing to the conduction of electrical signals. Intraventricular conduction disturbance may occur in patients with severe heart failure. In intraventricular conduction disturbance, a shift may occur in the sequence of conduction of electrical signals which cause the cardiac muscle to move. Due to this shift, there may be a portion which conducts early and a portion which conducts late in the cardiac ventricle which should conduct an electrical signal almost simultaneously. As a result, dyssynchrony occurs in the contraction of the cardiac wall, and blood cannot be sufficiently pumped out, resulting in heart failure. The cardiac resynchronization therapy is applied to such disturbance to help the pumping function of the heart by adjusting the sequence of conduction of electrical signals to the heart by artificially outputting electrical signals. More specifically, this therapy is achieved by embedding a pacemaker under the skin of the chest.
Such cardiac resynchronization therapy has already been applied to many patients, and dramatic improvements in symptom have been confirmed. On the other hand, it has been confirmed that about 30% of all the patients as heart failure cases have exhibited no improvements in symptom even upon application of this cardiac resynchronization therapy. This is because it cannot be accurately determined whether the cause of a heart failure case is systolic dyssynchrony. An application criterion for the cardiac resynchronization therapy is that the QRS width of an electrocardiographic waveform is more than 130 msec, and the left ventricular ejection fraction (EF) is less than 35%. This criterion is, however, met by even patients who have heart failure but have no systolic dyssynchrony.
Under the circumstances, therefore, there has been developed a technique of extracting only systolic dyssynchrony by a quantitative evaluation method using an ultrasonic diagnostic apparatus. As such a technique, for example, the technique disclosed in Jpn. Pat. Appln. KOKAI Publication No. 10-262970 is known, which detects the motion velocity of the cardiac muscle (cardiac wall) and computes/analyzes the motion velocity. According to this technique, the peaks of changes with time, e.g., changes in motion velocity or displacement, at a plurality of regions of the cardiac muscle can be automatically detected. The times taken to reach these peaks from a predetermined cardiac phase are calculated, and an ultrasonic image of the cardiac muscle is colored in accordance with the calculated times. Outputting the motion state differences of the overall cardiac muscle as a color image makes it possible to visualize the differences in motion timing between the respective regions of the cardiac muscle.
The following problems, however, arise in the above conventional techniques. According to the technique disclosed in patent reference 1, the motion velocity of the cardiac muscle is computed/analyzed. However, since velocity is an instantaneous physical quantity, a change in myocardial velocity with time does not always have a peak only in accordance with the contraction state of the ventricle. In a heart failure case, in particular, it is known that the myocardial velocity has many peaks, because the cardiac muscle moves abnormally. That is, it is difficult to perform stable evaluation from a motion velocity to determine which peak is a peak indicating significant ventricular contraction.
This relationship will be described concretely with reference to FIGS. 12 and 13. In each graph, the solid line represents a change in the velocity of a region ROI 1 designated on an ultrasonic image of a portion near the left ventricle with time, and the dotted line represents a change in the velocity of a region ROI 2 with time. Each graph indicates a change in velocity in one cardiac cycle and, more specifically, a change in velocity in the interval from early-systole to end-diastole which is delimited near an R wave. FIG. 12 shows a case wherein peak detection is performed in an area (a) including the early-systole interval. At this time, the peak of the motion velocity of the ROI 1 is detected at a position tp1, and the peak of the motion velocity of the ROI 2 is detected at a position tp2. If, however, peak detection is performed in an area (b) excluding the early-systole interval as shown in FIG. 13, the peak detection result totally differs from the above. Although the peak of the motion velocity of the ROI 2 is still detected at the position tp2, the peak of the motion velocity of the ROI 1 is detected at position tp3. The ultrasonic images in FIGS. 12 and 13 are shaded in accordance with the times taken to reach the peaks. In the case shown in FIG. 12, it is observed that the septum side contracts early, and the lateral wall side contracts late. In the case shown in FIG. 13, the opposite is observed.
According to the technique disclosed in patent reference 1, therefore, since the possibility of erroneously detecting a peak indicating significant contraction in a heart failure case is high, it is difficult to reliably detect systolic dyssynchrony.